Glass composition and substrate for information recording media comprising the same

ABSTRACT

A glass composition is disclosed which has a high modulus of elasticity (Young&#39;s modulus) and a high rigidity (modulus of elasticity/specific gravity) and is capable of being effectively inhibited from bending or vibrating. Also disclosed is a substrate for information recording media, which comprises the glass composition. The glass composition comprises the following components in terms of mol %: 40 to 60% SiO 2 , 8 to 25% Al 2 O 3 , 2 to 20% Li 2 O, 0 to 5% Na 2 O; provided that the content of R 2 O (R 2 O=Li 2 O+Na 2 O) is from 2 to 20%, 0 to 10% TiO 2 , 0 to 10% ZrO 2 , 5 to 25% MgO, 0 to 25% CaO and 0 to 6% SrO, provided that the content of RO (RO=MgO+CaO+SrO) is from more than 15 to 40%.

FIELD OF THE INVENTION

The present invention relates to a high-modulus glass composition. Moreparticularly, this invention relates to a glass composition suitable foruse as a substrate for information recording media, which is required tohave excellent surface smoothness and a high modulus. The presentinvention further relates to a substrate for information recordingmedia, which comprises the glass composition, and to an informationrecording medium.

BACKGROUND OF THE INVENTION

Information recording devices such as magnetic disks are always requiredto have a larger recording capacity and to attain a reduction in accesstime such as disk rotational delay. One possible means for satisfyingthe latter requirement is to heighten the rotational speed of a medium.

However, the media comprising a substrate currently in use are weigheddown by themselves and resonate considerably at an increased rotationalspeed. Eventually, the surface of such a medium comes into contact withthe head to cause an error or crushing. It is therefore impossible tonarrow the gap between the magnetic disk head and the recording mediumto or below a certain level, and this constitutes a serious obstacle toan increase in recording capacity.

For reducing the bending of a substrate medium and diminishing theresonance of the medium being rotated, it is necessary to heighten boththe modulus of elasticity (Young's modulus) of the substrate of themedium and the rigidity thereof which is the value obtained by dividingthe modulus of elasticity by the specific gravity.

The aluminum alloy which has been most commonly used as the substratesof magnetic disks has a modulus of elasticity of 71 GPa and a rigidityof 26 GPa·cm³/g. This conventional substrate material, having suchproperties, hardly copes with the trend toward higher rotational speedsof 10,000 rpm and above. In addition, it has become necessary toincrease the thickness of substrates made of the above material,although this goes against the current trend toward thickness reductionin disk substrates for device miniaturization.

In contrast, substrates made of a tempered glass are superior to thealuminum substrate in both modulus of elasticity and specific gravity.For example, a glass substrate obtained by subjecting a commerciallyavailable soda-lime glass to ion exchange in a molten potassium salt ison the market. This substrate has a modulus of elasticity of 72 GPa anda rigidity of 29 GPa·cm³/g.

Also known besides the above one is a glass substrate obtained bytempering commercially available Corning 0317. Although this substratehas a modulus of elasticity of 72 GPa and a rigidity of 29 GPa·cm³/g,these properties are still insufficient.

A high-rigidity substrate for information recording media which is madeof a material other than tempered glasses is on the market. Thissubstrate comprises a crystallized glass having a modulus of elasticityof 90 GPa and a rigidity of 38 GPa·cm³/g. However, this substrate, afterpolishing, inevitably has residual crystal grains projecting from thesurface because of the nature of the production process in whichcrystals are precipitated inside. Namely, this crystallized-glasssubstrate has a drawback that it is inferior in surface smoothness tothe substrates made of a tempered glass.

Consequently, in view of the expected future trend toward even higherrotational speeds in information recording devices and smaller thicknessin disk substrates, there is a desire for a glass composition which hasfurther improved properties, i.e., which has a high Young's modulus anda high rigidity, can be easily tempered, and gives a substrate havinghigh surface smoothness through polishing.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a glasscomposition which has a high modulus of elasticity (Young's modulus) anda high rigidity (modulus of elasticity/specific gravity) and is capableof being effectively inhibited from bending or vibrating when used asthe substrate of an information recording medium.

Another object of the present invention is to provide a substrate forinformation recording media, which comprises the glass composition.

The present invention has been achieved in view of the above-describedproblems of prior art techniques and the above-described requirements.

The present invention provides a glass composition comprising thefollowing components in terms of mol %: 40 to 60% SiO₂, 8 to 25% Al₂O₃,2 to 20% Li₂O, 0 to 5% Na₂O; provided that the content of R₂O(R₂O=Li₂O+Na₂O) is from 2 to 20%, 0 to 10% TiO₂, 0 to 10% ZrO₂, 5 to 25%MgO, 0 to 25% CaO and 0 to 6% SrO, provided that the content of RO(RO=MgO+CaO+SrO) is from more than 15 to 40%.

The glass composition preferably comprises the following components interms of mol %: 40 to 55% SiO₂, 10 to less than 20% Al₂O₃, 2 to 10%Li₂O, 5 to 5% Na₂O; provided that the content of R₂O (R₂O=Li₂O+Na₂O) isfrom 2 to 10%, 0 to 10% TiO₂, 0 to 10% ZrO₂, 2 to 25% MgO, 0 to 25% CaO,and 0 to 6% SrO, provided that the content of RO (RO=MgO+CaO+SrO) isfrom more than 15 to 30%.

The glass composition preferably has a rigidity as defined by (Young'smodulus)/(specific gravity) of 35 GPa·cm³/g or higher and a modulus ofelasticity as represented by Young's modulus of 95 GPa or higher.

Furthermore, the glass composition is preferably one which has undergonean ion exchange treatment in at least one molten salt containing ions ofpotassium, sodium, or both.

The present invention further provides a substrate for informationrecording media, which comprises the above-described glass compositionwhich has undergone the ion exchange treatment.

This invention still further provides an information recording mediumcontaining the substrate.

DETAILED DESCRIPTION OF THE INVENTION

The reasons for limitations of the components of the high-rigidityhigh-modulus glass composition of the present invention are explainedbelow. Hereinafter, unless otherwise indicated, all percents are bymole.

SiO₂ is the main component constituting the glass. If the proportion ofSiO₂ is lower than 40%, the glass has impaired chemical durability. Onthe other hand, if the proportion thereof exceeds 60%, the desiredmodulus of elasticity is not obtained. Consequently, the proportion ofSiO₂ should be from 40 to 60%, and is preferably from 40 to 55%.

Al₂O₃ is an ingredient which improves the modulus of elasticity andrigidity of the glass and increases the depth of a compression stresslayer formed by ion exchange. Al₂O₃ further serves to improve the waterresistance of the glass. If the proportion of Al₂O₃ is lower than 8%,these effects are insufficient. On the other hand, if the proportionthereof exceeds 25%, the results are an increased viscosity, an increasein liquidus temperature which is severer than the viscosity increase,and impaired meltability. Consequently, the proportion of Al₂O₃ shouldbe from 8 to 25%, and is preferably from 10 to less than 10%.

Li₂O, which is an ingredient to be replaced in ion exchange, serves toimprove the modulus of elasticity and rigidity of the glass and to lowerthe melting temperature of the glass to thereby enhance its meltability.If the proportion of Li₂O is less than 2%, the rigidity is insufficient.On the other hand if the proportion thereof exceeds 20%, the substratehas impaired weatherability and impaired acid resistance. Consequently,the proportion of Li₂O should be from 2 to 20%, and is preferably 2 to10%.

Na₂O, which is an ingredient to be replaced in ion exchange, serves tolower the melting temperature and the liquidus temperature to therebyenhance meltability. If the proportion of Na₂O exceeds 5%, the modulusof elasticity is decreased, and weatherability and acid resistance areimpaired. Consequently, the proportion of Na₂O should be 5% or less.

If the total amount of Li₂O and Na₂O (R₂O) is less than 2%, ion exchangecannot be conducted, and meltability is insufficient. On the other hand,if the total amount thereof exceeds 20%, the modulus of elasticity andrigidity are decreased. Consequently, the total amount of R₂O ispreferably from 2 to 20%.

TiO₂ is an ingredient which improves the modulus of elasticity,rigidity, and weatherability of the glass. However, if the proportionthereof exceeds 10%, the glass has an elevated liquidus temperature andimpaired devitrification resistance. Consequently, the proportion ofTiO₂ should be 10% or lower.

ZrO₂ is an ingredient which improves the modulus of elasticity,rigidity, and weatherability of the glass. However, if the proportion ofZrO₂ exceeds 10%, the glass has an elevated liquidus temperature andimpaired devitrification resistance. Consequently, the proportion ofZrO₂ should be 10% or lower.

MgO is an ingredient which heightens the modulus of elasticity,rigidity, and meltability of the glass. However, if the proportion ofMgO is less than 5%, its effect is small. On the other hand, if theproportion thereof exceeds 25%, the glass has an elevated liquidustemperature and impaired devitrification resistance. Consequently, theproportion of MgO is preferably from 5 to 25%.

CaO is an ingredient which heightens the modulus of elasticity,rigidity, and meltability of the glass. However, if the proportion ofCaO exceeds 25%, the glass has an elevated liquidus temperature andimpaired devitrification resistance. Consequently, the proportion of CaOis preferably 25% or lower.

SrO is an ingredient which heightens the meltability of the glass.However, if the glass contains SrO in a large amount, itdisadvantageously has an increased specific gravity. Consequently, theproportion of SrO should be 6% or lower.

If the total amount of MgO, CaO, and SrO (i.e., the amount of RO) is 15%or lower, the glass is insufficient in modulus of elasticity, rigidity,and meltability. If the total amount thereof exceeds 40%, the glass hasan elevated liquidus temperature and impaired devitrificationresistance. Consequently, the total amount of RO is preferably from morethan 15 to 40%, and more preferably from more than 15 to 30%.

Besides the ingredients described above, other ingredients may be addedin a total amount of up to 3% for the purposes of coloring, meltclarification, etc. Examples of such optional ingredients include As₂O₃,Sb₂O₃, SO₃, SnO₂, Fe₂O₃, CoO, Cl, and F.

This glass composition, which contains Li₂O and Na₂O, can be easily madeto have an increased fracture strength by immersing the composition inat least one molten salt containing ions of potassium, sodium, or bothat a temperature not higher than the distortion point of the glasscomposition to thereby interchange these ions and thus generate acompression stress on the surfaces of the composition.

When this glass composition is used as a substrate for informationrecording media, this substrate is less apt to bend or suffer resonantvibration because it has a higher modulus of elasticity and a higherrigidity than conventional substrates. Therefore, the recording mediumemploying this glass composition is especially suitable for use inrecording apparatuses of the high rotational speed type.

The present invention will be explained below in more detail byreference to the following Examples. However, the invention should notbe construed as being limited to these Examples.

EXAMPLES 1 TO 10

Ten compositions as examples of the glass composition of the presentinvention are shown in Table 1 together with properties thereof.

TABLE 1 Example 1 2 3 4 5 6 Component SiO₂ 41.8 42.0 45.0 40.0 47.0 54.0(mol %) Al₂O₃ 9.5 9.0 19.0 22.0 13.0 17.0 Li₂O 16.8 18.0 7.0 2.0 14.07.0 Na₂O 0.0 1.0 0.0 0.0 3.0 0.0 K₂O 0.0 0.0 0.0 0.0 0.0 0.0 MgO 12.517.0 13.0 18.0 8.0 11.0 CaO 18.0 8.0 12.0 14.0 10.0 7.0 SrO 0.0 1.0 0.00.0 0.0 0.0 ZrO₂ 0.0 3.0 4.0 2.0 5.0 4.0 TiO₂ 0.0 1.0 0.0 2.0 0.0 0.0SnO₂ 1.0 0.0 0.0 0.0 0.0 0.0 As₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 Specificgravity 2.77 2.74 2.65 2.64 2.74 2.67 (g/cm³) Modulus of elasticity 108111 105 103 107 100 (GPa) Rigidity 39 41 40 39 39 37 (GPa · cm³/g)Example Comparative Example 7 8 9 1 2 3 4 Component SiO₂ 48.5 55.5 59.065.0 45.0 71.6 67.0 (mol %) Al₂O₃ 14.0 14.0 8.0 6.0 21.0 0.9 10.3 Li₂O10.0 8.0 4.0 2.0 5.0 0.0 0.0 Na₂O 4.0 2.0 1.0 5.0 10.0 12.7 13.0 K₂O 0.00.0 0.0 0.0 0.0 0.5 2.3 MgO 6.5 12.0 14.0 3.0 6.0 6.0 5.1 CaO 6.0 2.52.0 11.0 10.0 8.4 0.5 SrO 3.0 1.0 0.0 0.0 3.0 0.0 0.0 ZrO₂ 5.0 5.0 8.00.0 0.0 0.0 0.0 TiO₂ 3.0 0.0 4.0 8.0 0.0 0.0 0.6 SnO₂ 0.0 0.0 0.0 0.00.0 0.0 0.0 As₂O₃ 0.0 0.0 0.0 0.0 0.0 0.0 1.2 Specific gravity 2.86 2.822.92 2.74 2.65 2.50 2.46 (g/cm³) Modulus of Elasticity 104 102 107 89 9072 72 (GPa) Rigidity 36 36 36 32 34 29 29 (GPa · cm³/g)

An explanation is first given on the Examples. Common raw materials forglass including silica, alumina, lithium carbonate, sodium carbonate,basic magnesium carbonate, calcium carbonate, potassium carbonate,strontium carbonate, titania, and zirconia were mixed together toprepare batches as base materials for substrate glasses so that thesebatches gave compositions respectively having the components shown inTable 1. Each batch prepared was held at 1,550° C. for 4 hours with aplatinum crucible and then poured on an iron plate. This glass was heldat 650° C. for 30 minutes in an electric furnace. Thereafter, thefurnace was switched off to allow the glass to cool to room temperature.Thus, sample glasses were obtained.

The specific gravity, modulus of elasticity (Young's modulus), andrigidity (Young's modulus/specific gravity) of each of the sampleglasses were measured or calculated in the following manners. Theresults obtained are shown in Table 1 above.

Modulus of elasticity was determined by the following method.

A sample glass was cut into a piece and each side thereof was subjectedto mirror polishing to obtain a platy sample having dimensions of5×30×30 mm. Each sample was examined for density by the Archimedesmethod. Furthermore, the modulus of elasticity of each sample wascalculated by the ultrasonic method using a sing-around oscillator.

The glasses of Examples 1 to 9 according to the present invention eachhad a Young's modulus exceeding 95 GPa and a rigidity of 35 GPa·cm³/g orhigher.

The glasses of Examples 1 to 9 were then immersed for 1 hour in a meltof an 80:20 salt mixture of KNO₃ and NaNO₃ heated at 380° C. to conducttempering. Thereafter, each tempered glass was examined with apolarizing microscope to determine the thickness of the resultingcompression stress layer.

In each of the glasses of Examples 1 to 9, a compression stress layerhaving a thickness of 50 μm or larger had been formed. The components ofthese glasses thus proved to be suitable for tempering.

APPLICATION EXAMPLE

The sample glass of Example 1 described above was cut into a ring formhaving an outer diameter of 95 mm and an inner diameter of 25 mm. Thisdisk was ground and polished and subsequently subjected to temperingunder the same conditions as the above and then to mirror polishing(surface roughness R_(a)≦1 nm; JIS B 0601-1994) to regulate thethickness thereof to 1.0 mm. Thus, a substrate for magnetic recordingmedia was obtained.

Using the thus-produced substrate for magnetic recording media, amagnetic disk medium was produced in the following manner.

Chromium, Co—Cr—Ta, and carbon were deposited on the substrate as anundercoat layer, recording layer, and protective layer, respectively, bysputtering. A lubricating layer was further formed thereon to obtain amagnetic disk medium.

The medium thus obtained was set in a closed type magnetic-disk driveand continuously rotated at each of 10,000 rpm and 12,000 rpm. In eithercase, the medium was found to be free from troubles such as headcrushing caused by substrate vibration, because the glass substrate hada high Young's modulus and a high rigidity.

COMPARATIVE EXAMPLES 1 TO 4

An explanation is given below on Comparative Examples 1 to 4. Thecompositions of Comparative Examples 1 to 4 are outside the scope of thepresent invention. In particular, the composition of Comparative Example3 is a general soda-lime glass composition.

Sample glasses were prepared in the same manner as in the Examples,except the following. In Comparative Example 4, the batch prepared washeld at 1,600° C. for 16 hours with a platinum crucible and then pouredon an iron plate. This glass was held at 650° C. for 30 minutes in anelectric furnace, which was then switched off to allow the glass to coolto room temperature to obtain a sample glass.

The specific gravity, modulus of elasticity (Young's modulus), rigidity(Young's modulus/specific gravity), and compression stress layerthickness of each of these sample glasses were measured or calculated inthe same manners as in the Examples The results obtained are shown inTable 1.

However, tempering was conducted in the following manners, as differentfrom that in Examples 1 to 9. The glass of Comparative Example 3 wasimmersed for 3 hours in a molten salt of KNO₃ heated at 380° C. toconduct tempering. The glass of Comparative Example 2 was immersed for16 hours in a molten salt of KNO₃ heated at 440° C. to conducttempering. A section of each of these samples was examined with apolarizing microscope in the same manner as in the Examples to determinethe thickness of the resulting compression stress layer.

The glasses of Comparative Examples 1 to 4 each had a Young's modulus of90 GPa or lower and a rigidity of lower than 35 GPa·cm³/g.

Further, the compression stress layer in each of the glasses ofComparative Examples 3 and 4 had a thickness of 11 μm and 39 μm,respectively, which were far smaller than 50 μm or more in Examples 1 to9, despite the higher melt temperature and the longer immersion periodsthan in the Examples.

As described above in detail, the following effects are brought about bythe present invention.

According to the present invention, a glass composition having a higherrigidity and a higher modulus of elasticity than conventional glassescan be obtained.

According to one preferred embodiment of the present invention, ahigh-rigidity high-modulus glass composition which can be easilyproduced and has satisfactory devitrification resistance andweatherability can be obtained because the range of the glass componenthas been narrowed.

According to another preferred embodiment of the present invention, alimited high-rigidity high-modulus glass composition is provided. Thisglass composition can have a higher rigidity and a higher modulus ofelasticity than the conventional glasses and conventional aluminumalloy, which have conventionally been used as a substrate for magneticdisk.

According to still another preferred embodiment of the presentinvention, tempering is easy as in general soda-lime glasses, making itpossible to form a surface compression stress layer on the glasscomposition to a larger depth.

According to another embodiment of the present invention, a glasssubstrate suitable for use as the substrate of an information recordingmedium can be obtained which comprises the above glass composition andhas a high rigidity, a high modulus of elasticity, and a high fracturestrength.

According to further embodiment of the present invention, a recordingmedium employing the above glass substrate having a high rigidity, highmodulus of elasticity, and high fracture strength can be obtained. Thisrecording medium can be rotated at a higher speed, bends less, and isless apt to suffer resonant substrate vibration. Hence, the gap betweenthe magnetic disk head and the recording medium can be narrowed, makingit possible to attain an increase in storage capacity and a reduction inaccess time. Therefore, the recording medium employing the glasscomposition is especially suitable for use in recording apparatuses ofthe high rotational speed type.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A glass composition consisting of the followingcomponents in terms of mol %: 40 to 55% SiO₂, 10 to less than 20% Al₂O₃,2 to 10% Li₂O, 0 to 5% Na₂O, provided that the content of R₂O(R₂O=Li₂O+Na₂O) is from 2 to 10%, 0 to 10% ZrO₂, 5 to 25% MgO, 0 to 25%CaO, and 0 to 6% SrO, provided that the content of RO (RO=MgO+CaO+SrO)is from more than 15 to 30%.
 2. The glass composition as claimed inclaim 1, which has a rigidity as defined by (Young's modulus)/(specificgravity) of 35 GPa·cm³/g or higher and a modulus of elasticity asrepresented by Young's modulus of 95 GPa or higher.
 3. A glasscomposition consisting of the following components in terms of mol %: 40to 55% SiO₂, 10 to less than 20% Al₂O₃, 2 to 10% Li₂O, 0 to 5% Na₂O,provided that the content of R₂O (R₂O=Li₂O+Na₂O) is from 2 to 10%, 0 to10% ZrO₂, 5 to 25% MgO, 0 to 25% CaO, and 0 to 6% SrO, provided that thecontent of RO (RO=MgO+CaO+SrO) is from more than 15 to 30%, wherein theglass composition has undergone an ion exchange treatment in at leastone molten salt containing ions of potassium, sodium, or both.
 4. Asubstrate for information recording media, which comprises a glasscomposition consisting of the following components in terms of mol %: 40to 55% SiO₂, 10 to less than 20% Al₂O₃, 2 to 10% Li₂O, 0 to 5% Na₂O,provided that the content of R₂O (R₂O=Li₂O+Na₂O) is from 2 to 10%, 0 to10% ZrO₂, 5 to 25% MgO, 0 to 25% CaO, and 0 to 6% SrO, provided that thecontent of RO (RO=MgO+CaO+SrO) is from more than 15 to 30%, said glasscomposition having a rigidity as defined by (Young's modulus)/(specificgravity) of 35 GPa·cm³/g or higher and a modulus of elasticity asrepresented by Young's modulus of 95 GPa or higher and having undergonean ion exchange treatment in at least one molten salt containing ions ofpotassium, sodium, or both.
 5. An information recording medium using thesubstrate as claimed in claim 4.